Infrared action spectroscopy and time-resolved dynamics of the OD–CO reactant complex

Abstract

The infrared action spectrum of the linear OD–CO reactant complex has been recorded in the OD overtone region near 1.9 μm using an infrared pump-ultraviolet probe technique. The pure overtone band of OD–CO (2νOD) is observed at 5148.6 cm−1 and combination bands involving the simultaneous excitation of OD stretch and D-atom bend are identified 160.0 and 191.2 cm−1 to higher energy. Band assignments and spectroscopic constants are derived from the rotationally resolved structure of the spectra. The change in the ground state rotational constant upon deuteration demonstrates that the H/D-atom of the hydroxyl radical points toward CO in the OH/D-CO complex. Direct time-domain measurements yield a lifetime of 37(4) ns for OD–CO (2νOD) prior to decay via inelastic scattering or chemical reaction. This is significantly longer than the laser-limited lifetime of ⩽5 ns observed for OH–CO (2νOH), and is attributed in part to the closing of a near-resonant vibration to vibration energy transfer channel upon deuteration. Vibrational predissociation of OD–CO (2νOD) proceeds by a vibration to rotation and/or translation mechanism that yields highly rotationally excited OD (v=1) fragments. Intermolecular D-atom bend excitation, which drives the structural transformation from the reactant complex to the transition state for reaction, results in a dramatic shortening of the lifetime to ⩽6 ns (laser-limited). Excitation of the D-atom bend also supplies sufficient energy to reopen the near-resonant vibrational energy transfer channel, resulting in minimal rotational excitation of the OD (v=1) fragments. Finally, a ground state binding energy for OD–CO of D0⩽456 cm−1 is established from the OD (v=1) product state distribution.